Noise in electrical systems and other types of systems such as electro-optic and electro-acoustic systems may disrupt both the amplitude and phase of signals. However, because many systems are relatively insensitive to fluctuations in amplitude, the fluctuations in phase (denoted as phase noise) are generally more problematic. For example, an oscillator may be designed to output a sinusoid at a desired frequency. Oscillators typically include some type of amplitude-limiting feature so that only phase noise will be a major noise contributor to the output sinusoid.
Because phase noise is such an important factor of overall noise, designers often desire a measure of the phase noise for a given system. Various approaches have been used to characterize phase noise. For example, amplifiers have been characterized by inputting a signal of known frequency into the amplifier and measuring a resulting amplified output in a spectrum analyzer. But the sensitivity of such an approach is limited by the relatively-poor sensitivity of the spectrum analyzer. Moreover, it is difficult to measure phase noise at frequencies close to the carrier frequency.
Unlike a spectrum analyzer, a phase-locked discriminator system has relatively good sensitivity and allows measurements close to the carrier frequency. However, the configuration of a phase-locked discriminator system is cumbersome and time consuming. Thus, an automated phase-locked discriminator noise test measurement system has been developed as described in U.S. Pat. No. 6,393,372 that alleviates the cumbersome nature of such systems. FIG. 1 illustrates an embodiment of such an automated system 1. A low-noise source 9 provides an input signal 11 for driving a unit-under-test (UUT) 3. UUT 3 may be any device for which a user desires a phase noise test measurement such as an amplifier, phase-shifter, diplexer or other suitable device or system of devices. UUT 3 receives the input from source 9 and processes it to provide an output signal 5. For example, if UUT 3 is an amplifier, output signal 5 would be an amplified version of input signal 11. Output signal 5 is amplified by variable amplifier 15 to provide an input signal 23 to a mixer 21. Source 9 also provides a version of input signal 11 to a variable phase-shifter 29. Variable phase-shifter 29 shifts input signal 11 by 90 degrees to provide a phase-shifted signal 25 to another input port of mixer 21. In this fashion, the “carrier” signal (input signal 11) is eliminated from a mixer output signal 41. To keep output signal 41 in the proper dynamic range of an analog-to-digital converter (ADC) 49, mixer output signal 41 is processed by a low-noise matching amplifier 43 to provide an output signal 42 to ADC 49.
To eliminate the carrier signal, the phase-shifted signal 25 must be in quadrature (shifted 90 degrees) with respect to the carrier. If quadrature is not established, a DC offset will be present in a digital output 44 from ADC 49. A controller such as a processor 55 monitors digital output 44 and controls phase-shifter 29 using a control signal 65 to maintain quadrature. The elimination of the carrier signal from low-noise source 9 also depends upon whether the carrier (input signal 11) and the phase-shifted version of the carrier (signal 25) are of equal power when entering mixer 21. Thus, analogous to the control of phase-shifter 29, processor 55 also controls variable amplifier 15 responsive to processing digital signal 44 using a control signal 67 to maintain equal powers for signals 25 and 23. These powers need not be maintained exactly equal but instead may merely be within a sufficient range of each other so that linear operation of mixer 21 is assured. Those of ordinary skill in the art will appreciate that variable amplifier 15 does not just amplify but may also attenuate responsive to control signal 67. For example, if UUT 3 is an amplifier, variable amplifier 15 will have to attenuate output signal 5 to keep signals 23 and 25 in comparative power equality. Processor 55 may also control low-noise matched amplifier 43 using a control signal 71 to maintain signal 42 in the proper dynamic range for ADC 49.
Having controlled the components for quadrature operation as just discussed, processor 55 assures that the carrier signal is eliminated from digital output 44 such that digital output signal 44 simply represents the phase noise. The phase noise injected by low-noise source 9 may be accounted for by a calibrating operation in which UUT 3 is removed and source 9 simply feeds amplifier 15 directly, although such a direct feed may occur through a delay line (not illustrated). The resulting phase noise in digital signal 44 during calibration may be stored in a memory associated with processor 55. Thus, during testing of UUT 3, processor 55 (or a spectrum analyzer associated with processor 55) may perform a Fourier analysis of digital signal 44 to determine the phase noise power produced by low-noise source 9. The measured phase noise may then be adjusted by the phase noise injected by source 9 to determine the additive phase noise supplied by UUT 3.
The phase noise measured in digital signal 44 depends upon the frequency of input signal 11 provided by source 9. For example, UUT 3 may be quite noisy at one frequency but less so at another. To measure phase noise across a range of frequencies, processor 55 may command source 9 to change the frequency of input signal 11 using a command signal 69, measure the resulting phase noise, change the frequency again, measure the resulting phase noise, and so on. Advantageously, such measurement is performed automatically and accurately with no manual intervention or tuning as would be necessary in conventional phase noise test measurement systems.
Although a phase-locked discriminator system 1 represents a dramatic advance in the art, certain challenges remain. For example, many factors are involved in properly biasing or driving a given component for optimum low-noise performance. To intelligently control these factors, U.S. patent application Ser. No. 11/134,546, filed May 20, 2005, the contents of which are incorporated by reference herein, discloses a system that includes a phase-locked discriminator system such that a component or system of components may be biased appropriately to achieve optimum low-noise performance. In one embodiment, this system may be used to bias an oscillator signal source. In that regard, it will be appreciated that the illustrated separation between UUT 3 and low-noise source 9 of FIG. 1 is merely a conceptual separation should UUT be a source UUT such as an oscillator. In such a case, the source is the UUT. It would thus be redundant to drive a source UUT such as an oscillator with a source—there is no need for an external carrier signal source for a source UUT.
Turning now to FIG. 2, an intelligent biasing system 200 for a source UUT 210 is illustrated. To analyze the phase noise of a source UUT 210, a delay line 236 forms a delayed version 240 of output signal 5 from source UUT 210. Note that in case of a perfect source that provides a sinusoidal output signal cos(ωt), the phase difference between arbitrary times t1 and t2 depends solely upon the delay period between these times. However, in a real world source, there will also be some phase noise that affects this phase difference. In general, it can be shown that the selection of the delay period affects the ability of a phase noise test measurement system to measure phase noise at smaller frequency offsets to the carrier signal frequency as well as the sensitivity of the phase noise measurement. As the delay provided by delay line 236 is increased, the ability to measure phase noise at smaller offsets from the carrier frequency is enhanced as well as the sensitivity. However, delay cannot be arbitrarily increased because attenuation through the delay line may become too severe and affect the measurement. Comparing FIGS. 1 and 2, it may be seen that the control and operation of the variable amplifier 15, mixer 21, low-noise matching amplifier 43, ADC 49, and phase-shifter 29 is the same. Thus, source UUT 210 provides a version of input signal 11 to phase shifter 29 analogously as discussed with respect to low-noise source 9 of FIG. 1. Variable amplifier 15 provides output signal 23 to mixer 21, which mixes this signal with a phase-shifted version 25 of carrier signal 11.
A controller such as a processor 205 controls amplifier 15 with control signal 67 to maintain linear operation of mixer 21. Processor 205 also operates to tune controllable variable(s) within source UUT 210 using a command signal 220. For example, a conventional source designer has no real way to bias components such as transistors for the best phase noise performance. In general, the conventional designer may simply guess that a certain bias point is best for low-noise operation. However, system 200 can intelligently set a controllable variable such as, for example, the bias voltage for a transistor by finding the bias voltage that results in the lowest phase-noise performance. Similarly, processor 205 controls the carrier frequency used by source UUT 210 using control signal 69 analogously as discussed with respect to low-noise source 9 of FIG. 1. The “best fit” method or any signal processing method capable of “best data fit” determination may be used to determine an optimum setting for the controllable variables. The control of a control variable (CV) such control signals 67, 69, 65, and 220 with regard to their effect on a measured variable (MV) such as phase noise is discussed further herein. In general, the controllable variables for the source UUT are set such that they result in an optimum additive phase noise performance.
Although the intelligent biasing system disclosed in U.S. application Ser. No. 11/134,546 gives a designer the ability to bias or tune controllable variables within a source UUT so as to result in optimum additive phase noise performance, it does not provide the underlying low-noise source configuration. In other words, given an existing source, it may be tuned using the intelligent biasing system, but one must first design the source. In general, ultra low-noise sources are very expensive, typically costing as much as $150,000 or more. Although such expensive sources may be advantageously tuned using an intelligent biasing system, such tuning only adds to their already prohibitive costs.
Accordingly, there is a need in the art for a low-noise yet low-cost source.